Direct Measurement of the C13(α,n)O

Abstract

nucleosynthesis yields up to the stellar surface, is called 13 the third dredge up. As early recognized, the creation 14 of a 13 C-pocket within the He-rich mantel, through the 15 reaction 12 C(p,γ) 13 N(β + ) 13 C, is a byproduct of these 16 recursive mixing episodes [2, 3]. Such a thin pocket (a 17 few 10 −5 M of 13 C) harbors one of the most important 18 nucleosynthesis sites in the Universe. During the period 19 between two TPs, the temperature attains about 90 MK 20 and 13 C is activated as a neutron source through the 21 reaction 13 C(α,n) 16 O. This process provides a relatively 22 slow neutron flux (≈ 10 7 neutrons/s/cm 2 ) for about 10 4 23 years each time. Starting from seed nuclei in the iron 24 region, this neutron flux slowly builds up heavy elements 25 along the line of stability [4]. This s-process (slow-neutron-26 capture) is responsible for the production of about half of 27 all the heavy elements (A≥ 90) in the Universe. 28 In order to constrain this important nucleosynthesis 29 process, the cross section of the 13 C(α,n) 16 O neutron 30 source needs to be known in the astrophysical energy 31 window (the Gamow peak) around E 0 = 150-230 keV 1 . 32 The available direct cross section data in the lower energy 33 interval 280 < E < 350 keV [5, 6] are affected by un-34 certainties ≥ 40%. Any effort of pushing direct reaction 35 measurements to lower energies is basically rendered futile 36 by the steep drop of the cross section and the presence of 37 natural and instrumental backgrounds. Extrapolation of 38 the cross section into the Gamow peak is further compli-39 cated by a broad state at E x = 6356(8) keV (J π = 1 2 + ) 40 just near the α-threshold in 17 O (S α = 6359 keV [7]). The 41 cross section evaluation in the Gamow peak requires a 42 careful matching between the tail of this near-threshold 43 state and the higher-energy experimental data. While 44 this state has been the focus of great experimental at-45 tention over the past years [8-12] its influence remains a 46 major source of uncertainty for the s-process [13] and the 47 need for more cross section data to fill the gap has been 48 frequently expressed [13, 14]. 49 In order to provide direct data at low energies to bet-50 ter constrain the cross section inside the Gamow peak 51 the LUNA collaboration has performed an intensive ex-52 perimental campaign at the deep-underground accelera-53 tor LUNA400 [15] inside the Gran Sasso National Lab-54 oratory (LNGS). The LNGS neutron background flux 55 (≈ 10 −7 n/cm 2 /s) [16]) is dominated by the natural ra-56 dioactivity of the surrounding rock and it is up to four 57 orders of magnitude lower than on the surface of the 58 Earth, improving the sensitivity over previous studies. 59 The experimental setup and the target analysis are de-60 scribed in detail in refs. [17-19], following is a brief sum-61 mary. The accelerator provided a He + beam on target of 62 up to 150 µA with α energies of 305-400 keV, correspond-63 ing to c.m. energies in the range 233-306 keV. Deposited 64 1 All energies in th...

Statistical Hauser-Feshbach model description of $(n,α)$ reaction cross sections for the weak s-process

“…EMPIRE [77], and NON-SMOKER [8]. Figure 5 shows the MACS results of the present work for 33 S, 59 Ni, and 71 Ge target nuclei in comparison with those from several other model calculations based on different assumptions on the nuclear properties and approaches in modeling the reaction mechanism. These include NON-SMOKER reaction cross sections data base [8], Evaluated Nuclear Data File (ENDF) [7], Joint Evaluated Fission and Fusion (JEFF) Nuclear Data Library [9], JENDL Library for Nuclear Science and Engineering [10], ROSFOND nuclear data [11], and Chinese Evaluated Nuclear Data Library (CENDL) [12,13].…”

“…The theory framework outlined in Sec.II is employed in model calculations for a set of nuclei already introduced in s-process network calculations of a massive star, including convective He burning core and shell C-burning [30]. Figure 1 shows the (n, α) reaction cross sections for the following nuclei: 17 O, 18 F, 22 Na, 26 Al, 33 S, 37 Ar, 39 Ar, 40 K, 41 Ca, 59 Ni, 65 Zn, and 71 Ge. For the sake of completeness, we present not only astrophysically relevant low-energy range of the cross sections but a full result.…”

“…[31,[36][37][38][39][40][41][42][43]. More recently (n, α) reaction has been measured and analyzed in the s-process branching point for nucleus 59 Ni, in view of considerable differences between model predictions and experimental data [40,44]. From the experimental side, novel techniques and detector systems have been developed to provide accurate new data [45,46].…”

“…The corresponding energy range known as Gamow window, determines the effective stellar energy region in which most charged-particle induced nuclear reactions occur [56,58]. For example, recent measurement of 13 C(α, n) 16 O reaction provided data inside the s-process Gamow peak [59]. Recently the first measurement has been performed in the Gamow window of a reaction for the gamma-process in inverse kinematics using reaction 76 Se(α, γ) 80 Kr [60], and using (p, γ) reactions, the cross sections have have also been explored in the high energy tail of the respective Gamow window [57].…”

Mixing and Magnetic Fields in Asymptotic Giant Branch Stars in the Framework of FRUITY Models

“…The capability of FRUITY Magnetic models of matching grain data can be understood by focusing on the trends of 88 Sr and 138 Ba isotopes. Previous studies using post-processing s-process models found that 13 C pockets with a flat 13 C profile and masses larger than a few 10 −4 M are needed to explain the bulk of mainstream SiC grains with negative 88 Sr/ 86 Sr and 138 Ba/ 136 Ba isotope ratios [151]. The action of magnetic-buoyancy-mixing results in deep penetration of a few protons, so that, when H-burning restarts, almost all the protons are captured by 12 C, producing 13 C but a small amount of 14 N. Therefore, the ensuing 13 C pocket is quite extended and with a low and rather constant 13 C abundance (see Figure 2), as required to explain Sr-Ba grain data [144,164,165].…”